Rotary anode for x-ray tubes

ABSTRACT

An x-ray tube anode of the rotary type has a target surface formed by a thin layer of tungsten/rhenium alloy, supported by laminates comprising first a tungsten alloy lamination comprising at least 70 percent tungsten by weight and behind this a molybdenum alloy lamination, the anode being made by powdered metal technology so the three components are integrated with a porosity of low value. The target surface is resistant to roughening and the anode is substantially free from the development of cracks even when heavily loaded.

United States Patent [191 Schreiner et a1.

ROTARY ANODE FOR X-RAY TUBES- Inventors: Horst Schreiner, Numberg; ErnstGeldner, Rosstal; Helmut Dietz, Nurnberg, all of Germany Assignee:Siemens Aktiengesellschaft,

Munich, Germany Filed: Mar. 1, 1973 Appl. No.: 336,963

Foreign Application Priority Data Mar. 13, 1972 Germany 2212058 US. Cl.313/330, 313/60 Int. Cl. H01j 35/10 Field of Search 313/60, 330

References Cited UNITED STATES PATENTS 12/1958 Schram 313/60 Sept. 17,1974 3,710,120 1/1973 Friedel ..3l3/60 3,731,128 5/1973 Haberrecker..3l3/60 Primary Examiner--l-lerman Karl Saalbach AssistantExaminer-Darwin R. Hostetter Attorney, Agent, or FirmKenyon & KenyonReilly Carr & Chapin [57] ABSTRACT 8 Claims, 3 Drawing FiguresBACKGROUND OF THE INVENTION To produce x-rays, an anode provides atarget against which an electron stream, or cathode rays, are focused,the target producing the x-rays but at the expense of substantialheating of the target and anode. The efficiency of the target materialin the production of x-rays is proportional to its atomic number, thehigh atomic number of tungsten, in combination with its high meltingpoint, making it particularly suitable for the target of an x-ray tube.

Overheating and target surface damage results if the electron streampower, or tube current, is increased too much in an effort to obtain anincreasing x-ray output. Therefore, rotary anodes are used having adisk-like central portion and a beveled peripheral target surface areawhich constantly presents a new target surface area as the anoderotates. The working surface area of a rotary anode may be said toresemble in reverse the inside contour of a dished saucer.

Heretofore such a rotary anode has been made in the form of a laminatehaving two laminations, the front lamination forming the target surfacebeing a tungsten/rhenium alloy and being of substantial thickness, andthe other lamination, or supporting or back-up layer, being a molybdenumalloy. The rhenium component retards target surface roughening, and likeother elements which might be used instead, is expensive.

Such a prior art rotary anode also is subject to cracking when subjectedto the high thermal stressing resulting from operation of the x-ray tubeat high power, as exemplified by a power in the area of I kw. The frontlamination is relatively thick, so the tungsten alloying componentcomprises an undesirably large portion of the anode.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a rotary x-ray anode that is an improvement on such prior artrotary anodes, particularly when operated under heavy loading.

According to the present invention, this object is attained by makingthe target surface layer thinner and using behind it two additionallayers, the first of which is composed of either pure tungsten or a hightungsten alloy containing at least 70 percent by weight, the other orback or third layer comprising a molybdenum alloy.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the presentinvention are illustrated somewhat schematically by the accompanyingdrawings in which:

FIG. 1 is a cross section through a first form;

FIG. 2 in a corresponding manner illustrates a second form; and

FIG. 3, again in cross section, illustrates a third form.

DESCRIPTION OF THE PREFERRED v EMBODIMENTS In all three figures of theabove drawings, the anodes are designated 11. They are all of circularconfiguration and each has a central mounting hole which is unnumberedbecause it is the usual rotary anode mounting arrangement. In eachfigure the target surface layer is 12, the second or intermediate anodelayer is 13, and the final or back layer of lamination is 14, the samenumerals being used in FIGS. 2 and 3 because the only difference is inthe anode cross-sectional shape.

In accordance with the invention, the first or target surface layer 12comprises a tungsten alloy consisting of from 30 to 20 percent by weightof at least one, or one or more, of a metal selected from the classconsisting of zirconium, hafnium, niobium, tantalum, rhenium, .osmium,or iridium, in addition to the tungsten. These alloying elements, of thetungsten alloy, causes the target surface of the layer 12, or x-rayproducing layer, to resist roughening and the development ofmicro-cracks in the path of the high energy electron beam focal spotwhich, as can be seen from the drawings, is located in the area marked15. This focal point is at the beveled peripheral portion of the anodeas required for the angular projection of the resulting x-rays.Preferably the tungsten alloy has the described alloying elementslimited to a range of from 5 to 15 percent by weight, with the balancebeing tungsten. Of the possible alloying elements described, rhenium isthe one usually used.

7 The second layer 13, or intermediate lamination, is eithersubstantially pure tungsten or a tungsten alloy containing at leastpercent tungsten by weight. To enhance the cold ductility and heatresistance provided by pure tungsten, alloys maybe included, alloyingelements such as niobium, tantalum and/or zirconium being useful forthis purpose providing they are not used in excess of 30 percent byweight of the alloy.

The third or back layer 14 of the new anode consists of a molybdenumalloy with the alloying component being at least one, or one or more,metals selected from the class consisting of titanium, zirconium,hafnium, niobium, tantalum, tungsten and rhenium. These alloyingelements are used in amounts of from 0.05 to 20 percent by weight,preferably 2 to 10 percent by weight, the balance being the molybdenum.

The thickness of the x-ray producing or target surface layer 12 mayrange from 0.05 to 1 mm., preferably from 0.1 to 0.7 mm.; the thicknessof the intermediate layer lamination 13 may range from I to 5 mm.,preferably from 1.5 to 4 mm.; and finally, the thickness of the back orthird layer 14 may range from 2 to 6 mm., preferably from 2.5 to 4 mm.The total anode thickness at its peripheral portion which travelsthrough the electron beam focal spot 15 may range from 5 to 12mm.,preferably 6 to 8 mm., and from this it can be seen that the foregoingdimensional values have reference to this portion of the rotary anode,the central portion being substantially thicker as shown in FIGS. 1 and2, or the anode thickness may be the same radially throughout its extentas indicated'by FIG. 2.

In all instances the rotary anode of this invention may be made bywell-known powdered metallurgy techniques. The powdered metals aremolded in a die by compressing three layers of the respective powderedmetal components, one on top of the other, into a solid body or compactwith a well defined beveled peripheral portion. The compacting pressuresmay range from 1,000 to 8,000 kg/cm Consolidation may be effected byprior art sintering. Thereafter in a hydrogen atmosphere or undervacuum, the consolidated sintered shape is heated to temperaturesbetween l,400 and 1,800C. The heated shape is then compressed, such asby hot hammer forging, to give the anode blank a high final density, orin other words, to free the shape from pores as much as possible. Theultimately compacted or forged anode has its target surface area, whichis the area defined by rotation of the anode through the area 15,surface ground to within the narrow tolerances required by x-ray tubeanode target surfaces in general. The heated sintered shape may beremoved from its protective environment for the short time required forthe hammer forging.

Specific examples of the present invention are providedby the following,it being understood that the cross-sectional shapes may be asexemplified by FIGS. 1 through 3.

Example 1 A layer of powder consisting of a mixture of molybdenum powderand another metallic powder, such as tungsten, is first poured into asteel mold to a height of 6 mm., for the formation of the back layer 14.On top of this is poured a layer of pure tungsten 6 mm. high to formthesecond or middle layer 13. Above this a layer of a powder mixture ofWRelO is poured to a thickness of 1.5 mm. for the formation of the x-rayproducing layer or target surface 12. The three layers of powder aresubjected to a pressure of 4,000 kg/cm to form a solid compact withwell-defined edge or peripherally beveled portions. Next, sinteringtakes place in an atmosphere of hydrogen at a temperature of between2,000 and 2,400C. for the duration of one hour. Following the sintering,the highest possible density is obtained by one or more compressionoperations, such as hammer forging, at temperatures of between l,500 andl,700C. The sintered part may be electrically inductively heated in aprotective gas and then exposed to air for a short period during thiscompacting or forging. The final porosity of the anode is less than 0.3percent.

Example 2 A WRelO powder alloy is first poured into a steel mold to aheight of 1.5 mm. for the formation of the upper or target layer 12. Ontop of this layer of WNb3 powder is poured to a height of5 mm. for theformation of the second or middle layer 13. On top of this is poured apowder mixture of Mo and an additional metallic powder. e.g., hafnium,to a height of 6 mm., to form the back layer 14. The three layers ofpowder are subjected to a pressure of 3,000 kg/cm to form a compressedsolid compact with well-defined edges or beveled portion. The sinteringis carried out in two steps, a preliminary sintering in hydrogen atl,000C. for 30 minutes, and a high sintering in vacuum at 2,000 to2,400C. for one hour. Following the sintering, the highest possibledensity is obtained by one or more compression operations at atemperature between l,400 and 1,700C., in a protective gas if possible.The final porosity of the rotary anode, possibly densified by hothammering, is less than 0.3 percent.

Example 3 The procedure is as in example 2, except for the compositionof the three layers, e.g., the x-ray-producing layer 12 consists of a0.2 mm. thickness of a WRelS alloy powder, the middle layer 13 of L5 mm.of a WRe3 alloy powder, and the back layer 14 of 4 mm. of a MoTa5 alloypowder mixture.

Example 4 Procedure is as in example 1, but the molybdenum layer ispoured only 4 mm. high and consists of a MoZr5 powder mixture. Thesecond layer of pure tungsten powder is filled to a height of 8 mm. Thethird layer of powder consists of Wlr 0.5, the pouring height being 1mm.

The rotary x-ray anode of this invention, because of the permissiblethinness of the target layer 12, permits considerable saving of rheniumcontent of the overall anode structure 1, as compared to previouslyknown composition anodes using a thicker WRe alloy as thex-ray-producing or target layer. With the known rotary x-ray anodes, therhenium content of the x-rayproducing layer is limited to between 3 and10 percent by weight, to save on cost. It is well known that a higherrhenium content, e.g., between 10 and 20 percent, leads to a less severeroughening of the x-ray-producing surface layer than rhenium contentsbelow 10 percent of rhenium by weight. Because of the three-layerstructure of the rotary anode according to the invention, rheniumcontents of, for instance, 15 percent by weight are possible, whilekeeping the total rhenium content of the entire rotary anode smallerthan does the prior art. The other elements which may be used instead ofrhenium are also expensive.

What is claimed is:

1. An x-ray tube rotary anode comprising a plurality of integratedlaminations of sintered powdered metal of which the front or firstlamination is a tungsten alloy; wherein the improvement comprises saidanode having a second lamination of tungsten or high-tungsten alloyhaving a thickness from about 1 to about 5mm. behind said firstlamination, and a third lamination of a molybdenum alloy behind saidsecond lamination.

2. The anode of claim 1 in which said first lamination is from about0.05 to about 1 mm. thick.

3. The anode of claim 1 in which said third lamination is from about 2to about 6 mm. thick.

4. The anode of claim 1 in which said first layer is from about 0.05 toabout 1 mm. thick, said second layer is from about 1 to about 5 mm.thick and said third lamination is from about 2 to about 6 mm. thick,said anode having a beveled peripheral portion forming an electron beamfocus target annulus and the latter being from about 5 to about 12 mm.thick overall.

5. The anode of claim 1 in which said first lamination is an alloyconsisting of tungsten and from about 3 to about 20 percent by weight ofone or more alloying metals selected from the class consisting ofzirconium, hafnium, niobium, tantalum, rhenium, osmium, or iridrum.

6. The anode of claim 1 in which said second lamination is a tungstenalloy consisting of by weight of at least percent tungsten with thebalance being one or more alloying metals selected from the classconsisting of niobium, tantalum and zirconium.

7. The anode of claim 1 in which said third lamination is a molybdenumalloy consisting of molybdenum and from about 0.05 to about 20 percentby weight of one or more alloying metals selected from the classconsisting of titanium, zirconium, hafnium, niobium, tantalum, tungsten,and rhenium.

8. The anode of claim 4 in which said first lamination is an alloyconsisting of tungsten and from about 3 to about 20 percent by weight ofone or more alloying metals selected from the class consisting ofzirconium, hafnium, niobium, tantalum, rhenium, osmium, or iridium, saidsecond lamination is a tungsten alloy consist-

2. The anode of claim 1 in which said first lamination is from about0.05 to about 1 mm. thick.
 3. The anode of claim 1 in which said thirdlamination is from about 2 to about 6 mm. thick.
 4. The anode of claim 1in which said first layer is from about 0.05 to about 1 mm. thick, saidsecond layer is from about 1 to about 5 mm. thick and said thirdlamination is from about 2 to about 6 mm. thick, said anode having abeveled peripheral portion forming an electron beam focus target annulusand the latter being from about 5 to about 12 mm. thick overall.
 5. Theanode of claim 1 in which said first lamination is an alloy consistingof tungsten and from about 3 to about 20 percent by weight of one ormore alloying metals selected from the class consisting of zirconium,hafnium, niobium, tantalum, rhenium, osmium, or iridium.
 6. The anode ofclaim 1 in which said second lamination is a tungsten alloy consistingof by weight of at least 70 percent tungsten with the balance being oneor more alloying metals selected from the class consisting of niobium,tantalum and zirconium.
 7. The anode of claim 1 in which said thirdlamination is a molybdenum alloy consisting of molybdenum and from about0.05 to about 20 percent by weight of one or more alloying metalsselected from the class consisting of titanium, zirconium, hafnium,niobium, tantalum, tungsten, and rhenium.
 8. The anode of claim 4 inwhich said first lamination is an alloy consisting of tungsten and fromabout 3 to about 20 percent by weight of one or more alloying metalsselected from the class consisting of zirconium, hafnium, niobium,tantalum, rhenium, osmium, or iridium, said second lamination is atungsten alloy consisting by weight of at least 70 percent by weight oftungsten with the balance being one or more alloying metals selectedfrom the class consisting of niobium, tantalum and zirconium, and saidthird lamination is a molybdenum alloy consisting of molybdenum and fromabout 0.05 percent to about 20 percent by weight of one or more alloyingmetals selected from the class consisting of titanium zirconium,hafnium, niobium, tantalum, tungsten, and rhenium, said laminationthickness prevailing throughout said electron beam focus target annulus.